Phase shift optical loop spectroscopy

ABSTRACT

The invention provides a method and apparatus for measuring one or more optical properties, such as absorbance and refractive index, of a test medium such as a gas, a liquid, or solid material. The method comprises providing a passive optical waveguide loop comprising the test medium, launching in the optical loop an intensity-modulated light at a reference phase, detecting a phase of said light along the optical waveguide loop, and comparing the detected phase of said light along the loop with the reference phase, wherein the comparison provides information about one or more optical properties of the test medium.

RELATED APPLICATIONS

This application claims the benefit of the filing date of U.S.Provisional Patent Application No. 60/552,709, filed on Mar. 15, 2004,the contents of which are incorporated herein by reference in theirentirety.

FIELD OF THE INVENTION

This invention relates to methods and apparatus for measuring opticalcharacteristics of a test medium or media. In particular, the inventionrelates to use of the phase shift of light in an optical loop to measureoptical characteristics of a test medium or media.

BACKGROUND OF THE INVENTION

Measurement of low optical losses of an absorbing medium, such as a gasor a molecular beam, may be performed by measuring the ring-down time,or decay time, of a light pulse as the pulse makes multiple passesthrough the medium. Such measurements may be carried out in a ring-downcavity consisting of two or more mirrors, between which the light pulseis reflected, and in which the absorbing medium under test is disposed.The cavity can also be used to characterize the mirrors when noabsorbing medium is present. See, for example, Romanini et al., 1993;Scherer et al., 1997; Berden et al., 2000; Lehmann, U.S. Pat. No.5,528,040, issued Jun. 18, 1996.

Cavity ring-down spectroscopy (CRDS) is well established as a gas phasemeasurement method. Recently CRDS was shown to be applicable toabsorption measurements on liquid samples, in which a high finessecavity was either filled entirely with a liquid sample (Hallock et al.,2002) or the liquid was contained in a cuvette (Xu et al., 2002).

Ring-down spectroscopy using optical fibers rather than a cavity wasattempted by von Lerber et al. (2002), who deposited highly reflectivecoatings onto both end facets of a 10 m optical fiber. Stewart et al.,(2001) inserted a gas phase absorption cell into a fiber-loop, leadingto very high transmission losses. These losses necessitated the use of afiber amplifier, and the sensitivity of measurements using such anactive loop depended strongly on the amplifier's temporal stability.

We have previously demonstrated a fiber-loop ring-down technique forcharacterizing low-loss processes in optical systems and forspectroscopy of minute liquid samples (Loock et al. U.S. Pat. No.6,842,548, issued Jan. 11, 2005), based on measuring the ring-down timeof a light pulse injected into the loop. Although extremely sensitive,limitations of that technique include slow data acquisition rate andhigh cost of optical components such as fast pulsed lasers.

SUMMARY OF THE INVENTION

According to one aspect of the invention there is provided a method formeasuring one or more optical properties of a test medium, comprising:providing an optical waveguide loop comprising a test medium; launchingin the optical waveguide loop intensity-modulated light at a referencephase; detecting a phase of said light along the optical waveguide loop;and comparing the detected phase of said light along the loop with thereference phase; wherein comparing the detected phase and the referencephase provides information about one or more optical properties of thetest medium.

In a preferred embodiment, the optical waveguide loop is passive. Inanother preferred embodiment, the optical waveguide is an optical fiber.In various embodiments, the optical waveguide loop is the test medium,or the optical waveguide loop comprises a capillary channel for saidtest medium.

In another embodiment, the test medium is exposed to an evanescent waveof light that is guided by the optical waveguide loop. In a furtherembodiment, the optical waveguide loop comprises a cladding, and thetest medium is in the cladding.

In one embodiment, the optical property is absorbance. In variousembodiments, the light has at least one wavelength selected frominfra-red (IR), visible, and ultra-violet. The light may have awavelength selected from about 200 nm and 2000 nm, preferably about 200nm to about 1700 nm. The test medium may be selected from a gas, amolecular beam, a liquid, and a solid material. In a preferredembodiment, the test medium is a liquid.

In another embodiment, the optical waveguide loop comprises asingle-mode optical fiber, and the method comprises launching in theoptical waveguide loop a single longitudinal mode of anintensity-modulated light; wherein the phase of the longitudinal mode isindicative of one or more optical properties of the test medium. In afurther embodiment, the method further comprises measuring intensity ofsaid longitudinal mode.

In another embodiment, the test medium comprises a mechanical sensor forreceiving a mechanical force, and the one or more optical properties ofthe test medium provide information about the mechanical force receivedby the mechanical sensor. The mechanical force may be selected fromstress and strain.

According to another aspect of the invention there is provided anapparatus for measuring one or more optical properties of a test medium,comprising: an optical waveguide loop comprising a test medium; anintensity-modulated light source for illuminating the loop with light ata reference phase; a detector for detecting a phase of said light alongthe loop; and an analyzer for comparing the detected phase of the lightwith the reference phase of the light; wherein the comparison isindicative of one or more optical properties of the test medium. Theanalyzer may output a result of the comparison, wherein the result isindicative of one or more optical properties of the test medium.

In one embodiment, the apparatus further comprises a device fordisplaying and/or storing and/or manipulating data corresponding to atleast one of said reference phase, said detected phase, and saidcomparison.

In a preferred embodiment, the optical waveguide loop is passive. Invarious embodiments, the optical waveguide loop is an optical fiber or asingle-mode optical fiber. In a further embodiment, the opticalwaveguide loop is the test medium. In some embodiments the apparatusfurther comprises a capillary channel for guiding the test medium tosaid light.

In another embodiment, test medium is exposed to an evanescent wave oflight that is guided by the optical waveguide loop. In some embodiments,the optical waveguide loop comprises a cladding, and the test medium isin the cladding. In further embodiments, the test medium or the opticalfiber comprises a grating.

In various embodiments, the optical property is absorbance, and in otherembodiments, light has at least one wavelength selected from infra-red(IR), visible, and ultra-violet. The wavelength may be between about 200nm to 2000 nm, preferably between about 200 nm to 1700 nm.

In one embodiment, the apparatus comprises a microfluidic device.

In another embodiment, the test medium comprises a mechanical sensor forreceiving a mechanical force, and the one or more optical properties ofthe test medium provide information about the mechanical force receivedby the mechanical sensor. The mechanical force may be selected fromstress and strain.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention are described below, by way of example,with reference to the accompanying drawings, wherein:

FIG. 1 is a block diagram of a phase shift fiber loop system accordingto the invention for measuring optical properties of an optical fiberand connector.

FIG. 2 is a plot showing a phase shift of the optical signal relative tothe (smooth) reference signal from the function generator, for the setupshown in FIG. 1.

FIG. 3 (a) shows a plot of phase angle as a function of concentrationfor a 26.4 m fiber loop, used to determine a phase angle difference ofφ₀=−36.3°, absorption A₀=0.60 in absence of1,1′-diethyl-4,4′-dicarbocyanine iodide (DDCI) dye, and gap between thefiber ends of d=42 μm; (b) shows concentration dependence of phase anglein capillary electrophoresis using a microcross. Here, φ₀=−23.3°, d=31μm and A₀=0.66 were determined from the fit for a L=65 m fiber loop.

FIG. 4 shows a plot of phase angle vs time for 1.0 and 2.0 mM solutionsof DDCI dye placed between two ends of a fiber, forming a fiber loop.The solutions were rapidly exchanged and the phase angle was measured.Measurements were taken every 100 ms and the time resolution of themeasurement was sufficient to monitor the time taken for the solution tobe completely replaced.

FIG. 5 is a plot of phase angle vs time showing transient absorptionpeaks due to absorption of DDCI in dimethylsulfoxide (DMSO), recorded bypushing the analyte through a 100 μm capillary using a syringe: theupper panel shows peaks at two concentrations; and the lower panel showspeaks for three different lengths of plugs of a 1 mM solution. There issignificant peak broadening because of the large size of the capillary(100/360 μm).

FIG. 6 is a plot showing capillary electrophoresis separation of twodyes, ADS 805 WS and ADS 830 WS dissolved in water (buffered by KH₂PO₃,pH˜8.0). 4.5 kV was used to elute the mixture.

FIG. 7 is a schematic diagram of an experimental setup used toinvestigate optical losses of a fiber optic cable in the 1.55 μmwavelength region by determining optical losses of radiation travellingthrough the fiber core independently from the losses by the cladding.

FIG. 8 is a plot of the phase angle φ as a function of the angularmodulation frequency Ω using the setup shown in FIG. 7, for broadbandand narrow band excitation sources. The optical decay constant τ and theoffset phase angle φ₀ were determined from the slope.

FIG. 9A is a schematic diagram of a fiber-optic strain sensor accordingto the invention.

FIG. 9B is a plot of the phase angle as a function of the load on thefiber-optic strain sensor of FIG. 9A.

FIG. 9C is a plot showing the time response of the fiber-optic strainsensor of FIG. 9A.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Engeln et al. (1996) measured the phase shift associated with an opticaldecay in cavity ring-down spectroscopy (CRDS) and observed that, due tothe greatly enhanced duty cycle, the accuracy of the technique comparedfavourably with pulsed CRDS. To our knowledge, phase shift CRDS(PS-CRDS) has been used only once since 1996, when Lewis et al. (2001)reported on the Δv=6 vibrational overtone transitions of differenthydrocarbons. DeMille et al. (2002) later compared these measurementswith overtone spectra obtained using conventional CRDS and intracavitylaser photoacoustic spectroscopy and noted that PS-CRDS yieldsabsorption cross sections about 35% higher than those obtained witheither of the conventional techniques, possibly indicating systematicerrors in the PS-CRDS system. The lack of interest in PS-CRDS is likelydue to its shortcomings, which include the inability to deal withmultiple exponential decays, mode-beating, sensitivity to electrical andoptical noise, and difficulty of measuring small phase angles in 1 MHzoscillations.

We nevertheless believe that the phase shift technique in general offersa major advantage over pulsed (time-resolved) CRDS, in that the timeresponse of measurements is vastly improved. We believe that the fasttime response of the technique is of particular relevance to liquidphase spectroscopy of very small samples that can be changed or cycledrapidly, where measurement time becomes the limiting factor in samplethroughput. Also, there may be other sources of optical loss that changequickly and can not readily be sampled using the conventional pulsedCRDS technique. Such losses could, for example, be due to fast chemicalprocesses or mechanical modifications to the cavity geometry.

This invention is based, in part, on the recognition that the advantageof the phase shift technique may be exploited when adapted to ameasurement method employing an optical wave guide loop. Prior to theinvention, such spectroscopic techniques could not be carried out onvery small liquid phase samples, and for gas phase measurements, thefast time response was not important enough to offset the drawbacks ofthe technique. Although Stewart et al. (2001) suggested that a phaseshift technique might be applied to a measurement method employing anoptical fiber loop, that suggested method used a microoptic gas cellhaving high loss (1 dB), and hence required an amplifier in the loop. Asa result, the technique suggested by Stewart et al. (2001) did notinvolve a passive loop. Moreover, the suggested technique would becomplex and expensive to implement, and would be vulnerable tocomplications such as gain control and amplifier stability associatedwith an amplified loop.

According to a broad aspect of the invention, there is provided a methodof measuring one or more optical properties of a test medium bymeasuring the phase shift of light traveling around an optical waveguideloop and through the test medium. The invention provides a method bywhich the phase shift of light in a waveguide loop can be used incharacterizing the optical properties of a test medium. Preferably, theoptical waveguide loop is passive, meaning that the loop does not have adevice (e.g., an amplifier) for amplifying light.

As used herein, the terms “change in phase”, “phase change”, “phaseshift”, and “phase angle difference” are equivalent and refer to achange in the phase φ of an optical signal as a function of the testmedium through which the signal travels. A change in phase may beconsidered as a temporal shift in a light signal waveform (the waveformhaving a period of 360 degrees) after passing through a test medium,relative to a reference. The reference may be, for example, the phase ofthe light signal prior to passing through the test medium. Forconvenience, the reference phase (e.g., phase of the light signalentering the optical waveguide) may be designated as 0 degrees, and thephase change of the light signal after passing through the test medium(e.g., phase of the light signal exiting the optical waveguide) may beexpressed relative to 0 degrees.

As used herein, the term “test medium” is intended to mean any medium ormaterial the optical properties of which can be measured in accordancewith the invention. The test medium is exposed to at least a portion ofthe light that is guided by the optical waveguide, wherein that portionof light is either within the waveguide, or outside of the waveguide(i.e., the evanescent wave). Examples of test media include, but are notlimited to, the optical waveguide loop itself, a portion of a secondoptical waveguide inserted into the loop, a modified optical waveguide,an optical connector or an optical device (e.g., a grating or filter), asample of a gas, liquid, or solid material (e.g., a film or coating,such as a solid or liquid film deposited on the facet end of an opticalfiber), a molecular beam, or a stationary test medium. Samples of gasesand liquids may be introduced into the loop using a small conduit forconducting the sample therethrough, the conduit intersecting the loop ina manner that allows all or a portion of the light to pass through thesample. For example, the conduit may be a tube, such as a capillary tube(i.e., a “capillary”), or a flow channel, which may also be referred toas a capillary channel or a capillary flow channel. The flow channel maybe etched or machined into a substrate. It will be appreciated that flowof a sample through such a conduit may or may not involve capillaryaction. Where flow does not involve capillary action, flow may beestablished using any technique known in the art, such as a pump, anelectrical and/or chemical gradient, a pressure differential, or thelike. A test medium may also be introduced to the optical waveguide loopso as to intercept at least a portion of the evanescent wave. In thecase where an optical fiber is used for the optical waveguide, this maybe achieved by, for example, modifying a portion of the fiber claddingto accommodate a test medium. For example, a portion of the fibercladding may be removed to expose a test medium to the evanescent wavethat resides in the cladding close to the interface of the cladding andthe fiber core.

As used herein, the term “optical properties” is intended to mean anyproperty of a medium that is light-dependent. Examples of opticalproperties are absorbance, scattering efficiency, refractive index,evanescent wave spectrum, and optical loss. Optical properties areindicative of, or related to, physical and/or chemical characteristicsof a medium (e.g., density, structure (such as 1-, 2-, or 3-dimensionalstructure)). Thus, in accordance with the invention, one or more opticalproperties of a medium is/are indicative of one or more physical and/orchemical characteristics of the medium.

As used herein, the term “optical waveguide” is intended to encompassany conduit for light. An optical waveguide according to the inventionis capable of being formed into or provided as a continuous loop, e.g.,by joining the two ends of the waveguide together, such that a lightsignal launched in the waveguide travels around the loop repeatedly.Thus, as used herein, the term “optical waveguide loop” refers to a loopmade of an optical waveguide. The loop is continuous insofar as itprovides a continuous path for a light signal travelling therethrough;however, the loop may have an opening into which a test medium may beinserted. Examples of optical waveguides are optical fibers, such asthose having a solid core, hollow core (i.e., capillary fiber), orliquid core, and waveguides based on high refractive index fluids.Optical waveguides can also be prepared on a substrate such as glass orpolymeric material, for example, in embodiments where the inventioncomprises a microchip. Where optical fiber is employed, such fiber maybe selected from commercially available fibers, including multi-mode andsingle mode fibers. The two ends of waveguides such as optical fibersare joined using splice connectors, such as any commercially availableconnector, fusion spliced connections, or any other suitable techniqueknown in the art. Preferably, such fibers and connections have lowtransmission loss (e.g., absorbance, geometrical mismatch, scattering).In this regard, waveguides based on high refractive index fluids areadvantageous in that such connectors are not required.

In accordance with the invention, the optimum length of the opticalwaveguide loop depends on the desired measurement sensitivity anddetection limit. Generally, as optical losses within the loop decrease,the loop may be made shorter. For very small losses, the loop ispreferably as short as possible. In a typical measurement scenario, suchoptical losses would be caused by absorption of the test medium. It isexpected that for certain applications an entire apparatus according tothe invention may be fabricated on a microchip. In practice, the minimumlength of the loop may be limited by factors that contribute to loss ofthe light signal, such as a small radius of the bend in the waveguide(loss increases as radius decreases), high loss of a waveguide splice(e.g., a fiber optic splice connector), or high absorbance of a testmedium. Loop length is discussed in detail in Jakubinek et al. (2004).It should be noted that in forming the loop, the optical waveguide canbe “wound” into any shape, as may be required for compactness, etc., ofthe loop. This is of relevance especially when long loops are required.

It is preferred that the optical waveguide loop is a passive loop. Asused herein, the term “passive loop” refers to a loop that does not havea device (e.g., an amplifier) for amplifying light.

In accordance with the invention, the light signal may be of anywavelength from about 2000 nm to about 200 nm (i.e., infra-red (IR) toultra violet (UV)), preferably from about 1700 nm to about 200 nm.However, use of wavelengths longer than 1700 nm may be problematic dueto the high transmission losses in typical waveguide materials (e.g.,silica). Use of UV may also be problematic because of the degradativeeffects of UV light on optical materials, and comparatively high losses(e.g., 1% per m of optical fiber). However, UV is of particular interestin chemical, biochemical, biological, and environmental studies, becausemany compounds and substances of interest absorb in this wavelength. Insome embodiments of the invention the light signal has a narrowbandwidth (e.g., a single colour of light), whereas in otherembodiments, the light signal is wide band (e.g., white light). Suitablelight sources are those light sources capable of being modulated at arate that is comparable to 1/τ, where τ is the optical decay constant ofthe waveguide loop. The light source may be, for example, a laser, alaser diode, or a light emitting diode (LED). In embodiments employing aspectroscopic approach wherein ring-down time as a function ofwavelength is sought, a tunable laser can be employed, and such laserswept to produce laser radiation over a range of wavelengths. The lightsignal may be coupled into the waveguide using any conventional approachor device, such as, for example, a directional coupler. However, in thecase of optical fiber, light may be coupled into the fiber simply byilluminating the fiber. When using a 1 W laser, the inventors achievedgood coupling of light into an optical fiber by first focussing thelaser light into a power delivery fiber, and then connecting this fiberto the loop using a drop of DMSO solvent, which acted as index matchingfluid. When using a 10 mW infrared laser at 1600 nm, low loss wasachieved using a 99.5%:0.5% directional coupler, despite the fact thatthe coupler introduced a considerable (4%) insertion loss of light perpass around the loop. Using evanescent coupling (Polynkin et al. 2004)these insertion losses were further reduced and coupling efficiency wasfurther optimized.

One embodiment of the invention is shown in FIG. 1. In this embodiment,a function generator 2 modulates the intensity of light from a sourcesuch as a laser 4. The modulated light signal is coupled into an opticalwave guide, in this case an optical fiber 6, which is formed into a loopusing a fiber splice connector 8. As shown in the inset, the opticalfiber comprises a core 10 and cladding 12. The light signal travelingthrough the fiber loop is detected using a photon detector 14. Thedetected light may be displayed on a suitable device such as anoscilloscope, while the phase angle difference between the lightentering and exiting the fiber 6 may be determined, for example, by alock-in amplifier 16 or a ratiometer. The data may be stored in and/oranalyzed by a computer. By measuring the phase angle with respect to asuitable reference such as the phase of the incoming modulated light(see FIG. 2), various loss mechanisms of the test medium can becharacterized. Advantageously, such losses are largely independent ofpower fluctuations of the light source. Thus, unlike conventional singleor multipass-type devices, the invention is not sensitive to theintensity of the input light signal, to the input coupling efficiency ofthe light signal, or to drift of the light signal power with, forexample, time, temperature, or wavelength. The method of the inventionis not very sensitive to laser alignment, and a long loop can beprovided to allow for spatially separate illumination and detectionregions.

In a preferred embodiment, the invention comprises launching a modulatedcontinuous light signal into the optical loop, and recording the phaseangle over a time constant corresponding to a number of oscillationcycles (i.e., periods or wavelengths) of the modulated light signalbeing averaged. Use of a larger time constant increases the number ofoscillation cycles averaged, and hence provides a more accurate measureof the phase angle difference. Use of a lock-in amplifier convenientlyprovides for adjustment of the time constant.

The embodiment shown in FIG. 1 is suitable for applications such ascharacterizing loss processes in fiber optic transmission. For example,the method can be used to accurately determine the absolute transmissionspectrum of an optical fiber and of the fiber connector, as well asother optical properties such as refractive index, evanescent wavespectrum, and optical loss. Further, the deformation (e.g., strain) of afiber can be evaluated by determining the effect of the deformation onone or more such optical properties of the fiber. Deformation may becaused by a mechanical force (e.g., stress), and/or by physical factors(e.g., temperature, pressure) acting on the fiber. For example,deformation may comprise bending the fiber, with a smaller radius of thebend associated with greater deformation, and hence greater changes ofoptical properties of the fiber.

In one embodiment, the invention is used to measure one or more opticalproperties (e.g., absorbance) of a test medium, using a short opticalpath length through the test medium (e.g., a path length less than about100 μm, preferably about 1 to 10 μm). A short optical path length can beachieved by using a very small capillary channel to introduce the testmedium (such as a liquid or a gas) into the loop, for example. It willbe appreciated that this embodiment requires only very small samplevolumes of test medium (e.g., in the order of picoliters).

In particular, the invention is advantageously used in spectrometry ofsmall volumes of test media such as fluid (i.e., liquid or gas) samples.It is desirable to measure very small samples of a substance,particularly when the substance is expensive, rare, or toxic. However,previously-known cavity techniques are not conveniently applied tomeasurement of very small samples because of the larger samplequantities generally required. Further, use of small sample volumes inaccordance with the invention makes possible rapid flushing of thechannel, and hence a high repetition rate for measurement of subsequentsamples (e.g., less than 1 s for a measurement). Previously known cavitytechniques cannot provide such rapid flushing of samples because of thelarge samples required, and hence cannot provide rapid measurement ofsuccessive samples. Previously known ring-down spectroscopy techniques,even when applied to measurement of small samples in capillary channels,cannot provide rapid measurement of successive samples because of theconsiderable time required for data acquisition and processing. Thus,none of the above-mentioned techniques provides rapid measurement ofoptical properties of very small fluid samples of test media.

The phase shift optical loop method of the invention provides for anextremely sensitive and rapid absorption spectroscopic technique, and assuch it is suitable for numerous applications, as exemplified by theembodiments described below. It is noted that, in contrast to most otherabsorption measurement techniques, the sensitivity of the phase anglemeasurement (i.e., the change of phase angle with concentration change)is larger for weak absorption processes than for strong absorptionprocesses. Therefore, the invention is well suited to weak absorbers,dilute samples, and/or short absorption path lengths.

In another embodiment, the phase shift due to optical losses of anevanescent wave is used for example to detect the presence of one ormore compounds, or to measure the absorption spectrum of one or morecompounds. This embodiment takes advantage of the fact that significantoptical energy resides in the cladding close to the interface with thecore. In such embodiment, the fiber cladding on a section of the loopcan either be removed, coated, replaced (e.g., with a chemicallymodified polymer, such as a silicon-based polymer), or modified (e.g.,chemically), to permit detection and recording of the evanescent waveabsorption spectrum produced by a compound(s) exposed to the evanescentwave. In particular, solid phase micro extraction (SPME) may be used, inwhich a polymer coating provides enrichment of the analyte (e.g., by2000 times or more) through extraction of the analyte into the polymer,may be used. The partitioning coefficients that govern the efficiency ofsolid phase microextraction vary depending on the class of chemicals andthe polymer matrix. For example, Table 1 lists a number ofsiloxane-based polymers and corresponding types of analytes for whichthey are suitable, which may be used in accordance with the invention.Partitioning of a molecule of interest into the cladding changes itsoptical properties (for example, refractive index, optical absorbance),resulting in a change in the reflection efficiency of the cladding andits optical losses. Effects of such changes on the evanescent wave canbe measured using the phase shift techniques of the invention.

TABLE 1 Examples of siloxane-based polymers suitable for solid phasemicro extraction (SPME) polymer coatings on optical fibers. SPME CoatingApplication 100 μm polydimethylsiloxane For Volatiles 7 μmpolydimethylsiloxane For Nonpolar High Molecular Weight Compounds 85 μmpolyacrylate For polar semivolatiles 30 μm polydimethylsiloxane ForNonpolar Semivolatiles 65 μm For Volatiles, Amines, andpolydimethylsiloxane/divinylbenzene Nitroaromatic Compounds 65 μmCarbowax/divinylbenzene For Alcohols and Polar Compounds 60 μm ForAmines and Polar polydimethylsiloxane/divinylbenzene Compounds (HPLC useonly) 50 μm Carbowax/templated resin For Surfactants (HPLC use only) 75μm Carboxen/polydimethylsiloxane For Gases and Low Molecular WeightCompounds 65 μm For Volatiles, Amines, andpolydimethylsiloxane/divinylbenzene Nitroaromatic Compounds 50/30 μmdivinylbenzene/Carboxen For Flavor Compounds (Volatiles andSemivolatiles) 85 μm Carboxen/polydimethylsiloxane For Gases and LowMolecular Weight Compounds 70 μm Carbowax/divinylbenzene For Alcoholsand Polar Compounds 100 μm polydimethylsiloxane For Volatiles 50/30 μmdivinylbenzene/Carboxen For Odor Compounds

In another embodiment, there is provided a method of measuringpolarization-dependent loss using pulsed polarized laser light as asource and a polarization-maintaining fiber for the loop.Polarization-dependent loss is an important quantity in thetelecommunications industry; however, such measurements are difficult toundertake with currently available technology.

According to another aspect of the invention there is provided anapparatus for measuring one or more optical properties of a test mediumby measuring the phase angle difference of the modulation of lightguided by the waveguide loop and passing through the test medium,relative to a reference phase angle. An example of such an apparatus isan absorbance detector.

In accordance with this aspect of the invention, the loop has a testmedium introduced therein. The test medium is a material for whichoptical properties are to be measured. For example, where optical fiberis employed for the optical loop, the medium used for index matching inthe fiber-splice may be replaced with a test medium such as water,organic solvent, etc. Typically, such test medium will have a refractiveindex different from the refractive index of the fiber core. In such anembodiment, the space between the two fiber ends acts as a Fabry-Perotcavity. The loss processes are then determined by the refractive indexof this cavity with respect to the fiber as well as by the modes presentin the fiber. It is therefore necessary to accurately determine the modestructure of the Fabry-Perot cavity and its change as a function of therefractive index of the cavity medium. Maintaining a stable modestructure in a conventional cavity ring-down laser absorptionspectroscopy experiment is challenging, since the mirrors are typicallyspaced by tens of centimeters and the laser pulse coupled into thecavity contains a large number of modes. In this embodiment, however,the loop substantially simplifies the measurement of the cavity modes ifa single mode waveguide is used.

In one embodiment, the invention provides an absorption detector whereina test medium for absorption measurement is introduced into the opticalpath of the optical loop. This can be accomplished by providing the testmedium in, for example, a capillary tube or channel or a flow channel,appropriately interfaced with the optical loop. For example, dependingon the dimensions of the optical waveguide and the capillary, flowchannel, capillary channel, or the like, the latter may either intersectthe optical waveguide, or it may pass through the waveguide, via, forexample, a hole through the waveguide. Further, at least a portion ofthe optical waveguide loop may be incorporated into a chip, such as, forexample, a microfluidic device. For example, where optical fiber isemployed, the splice connector may be replaced with such a microfluidicdevice (e.g., a “lab-on-a-chip” device). Such devices are provided withchannels having cross-sections in the order of microns, for carryingsmall amounts of analyte solution. The solutions can be separated intotheir solutes in the channels. A microfluidic device thus provides awell-defined small gap between the waveguide ends. The waveguide loopintersects one such channel, thereby forming part of a sensitive,selective absorption detector. The detection limit for such a device wasexperimentally determined to be about e[l/mol m]*c[mol/l]d[m]=10⁻⁵. Astrongly absorbing molecule (e.g., e=10⁶ l/mol m) can therefore bedetected at concentrations of several micromoles per liter. Improvementof the detection limit may be achieved through, for example, using alower base loss fiber connector and a low loss fiber, or by using alarger time constant for the phase angle measurement.

In a variation of this embodiment, polarization-maintaining fibers andoptically active analytes are used, such that small quantities ofabsorbing media can be detected in a small absorption cell.

In another embodiment the fiber loop is made of single-mode opticalfiber and the excitation laser has a bandwidth that is comparable to thespectral width of each mode. The fiber loop has a mode structuredependent upon the length of the loop and the diameter of the core. Byselectively exciting a single longitudinal mode of the loop, theintensity of light inside the loop and of the emitted light can beincreased, thereby reducing the time needed for averaging theoscillating signal.

In another embodiment optical properties such as the refractive indexcan be measured by selectively exciting a single longitudinal mode in afiber loop made of single-mode optical fiber, using a narrow bandwidthlaser and by tracking the wavelength of the longitudinal mode. The fiberloop has a mode structure dependent on the length of the loop and thediameter of the core. The wavelength position of each mode depends onthe refractive index of the Waveguide material; therefore, changes inthe refractive index may be tracked by monitoring the emitted intensitytogether with the phase angle as a function of wavelength.

In another embodiment, the fiber loop is adapted for measurement offorces (e.g., stress) and/or physical factors (e.g., temperature,pressure) that result in deformation (e.g., strain) of the fiber, bymeasuring the effect of such deformation on one or more opticalproperties of the fiber. Deformation, such as bending of the fiber, mayalter one or more optical properties, with a smaller radius of bendassociated with greater changes in the optical properties of the fiber.For example, a portion of the fiber maybe interfaced with suitablehardware so as to provide a mechanical strain sensor, as exemplifiedbelow.

The contents of all cited references are incorporated herein byreference in their entirety.

The invention is further described by way of the following non-limitingexamples.

WORKING EXAMPLES Example 1 Phase Shift Fiber Loop Spectroscopy

Introduction

Phase shift optical loop spectroscopy uses an intensity-modulated lightsource to pump the optical loop. This modulated pumping results in atime-varying light intensity in the optical loop, and hence the amountof light scattered from the fiber is also modulated in time. As will beshown below, the modulation frequencies of the incoming and emittedlight are identical and the phase angle between the modulated pumpinglight signal and the light scattered from the fiber provides an accuratemeasure for the optical loss in the loop, without the need for extendedaveraging and exponential fitting.

In these phase-angle ring-down measurements, a continuous wave (cw)laser beam is intensity modulated in time. This can be done eitherinternally or externally by an electro-optical modulator. The timedependence of incoming intensity isI _(in) =I _(0[)1+α sin(Ωt)]  (1)where α≦1 is the modulation depth and Ω=2πf is the angular modulationfrequency.

When such a modulated beam is injected into the loop, the energy densityin the loop, and hence the light intensity emitted by or scattered fromthe loop will be modulated with the same frequency, Ω, but will be phaseshifted with respect to the input signal (Engeln et al., 1996).

In Equation (2)

$\quad\begin{matrix}\begin{matrix}{{I(t)} = {\frac{1}{\tau}{\int_{0}^{t}{{I_{0}\left\lbrack {1 + {\alpha\;{\sin\left( {\Omega\; t^{\prime}} \right)}}} \right\rbrack}{\exp\left( {- \frac{t - t^{\prime}}{\tau}} \right)}{\mathbb{d}t^{\prime}}}}}} \\{= {I_{0}\left\{ {1 + {\frac{\alpha}{\sqrt{1 + {\Omega^{2}\tau^{2}}}}{\sin\left\lbrack {{\Omega\; t} - {\arctan\left( {\Omega\;\tau} \right)}} \right\rbrack}}} \right\}}}\end{matrix} & (2)\end{matrix}$where τ is the ring-down time of the cavity, it is assumed that thetransit time of light in the cavity is short compared to the ring-downtime and the modulation period. From the above equation, the phase shiftφ can be given asφ=−arctan(Ωτ)  (3)and the modulation depth of the emitted light is

$\begin{matrix}{\alpha^{\prime} = \frac{\alpha}{\sqrt{1 + {\Omega^{2}\tau^{2}}}}} & (4)\end{matrix}$

While the ring-down time can in principle be determined from either themodulation depth or the phase shift; only phase shift measurements havethe potential to yield comparable sensitivity, detection limit, and timeresponse to conventional time-resolved ring-down spectroscopy.

Methods

The experimental setup consisted of a laser diode (JDS UniphaseSDL-2372-P1, 810 nm±3 nm, max. 2 W) current-modulated at frequenciesaround 200 kHz. The laser output was delivered by 1 m of multimode fiberwhich was coupled to the fiber-loop (Fiber-Tech, Optica, AS400/440IRPI,26 m) with the aid of a drop of dimethylsulfoxide (DMSO). The couplingefficiency of such an arrangement is low (˜10⁻⁵), but can readily beimproved using commercial fiber-fiber couplers if necessary. Aphotomultiplier tube (PMT; Hamamatsu 950) was placed at a differentlocation along the loop and monitored the light intensity in the loop bydetecting photons scattered from the fiber core and cladding. Inpreliminary experiments the ends of a 400/440 μm optical fiber werecoupled using an x-y-z translation stage to form a loop with low opticallosses. The gap between the fibers was filled with either water or DMSOcontaining variable amounts of dye. Since the refractive index of DMSO(n=1.4787) is close to the refractive index of the fiber core (n=1.457),the solvent acted as index-matching fluid and nearly eliminated the backreflection at the fiber-solution interface. The lower refractive indexof water had a surprisingly small effect on the coupling efficiencybetween the fiber ends, possibly due to a focussing effect at thefiber-water interface. The alignment was optimized and characterizedusing a microscope.

The PMT signal was fed into a fast lock-in amplifier (Stanford ResearchSystems SR 844) and referenced to the driving current of the laserdiode. To reduce radio frequency interference, all the cables wereshielded. The size of the gap between the fiber ends was adjusted toabout 10% of the diameter of the fiber core and was experimentallydetermined to be about 42 μm.

To get the highest sensitivity in the phase-angle measurement, theangular modulation frequency was set to around Ω=1/τ, i.e., to a phaseangle of about φ=45° (see Equation 3). Due to the inherent time delaysin cables and electronic components, an offset phase angle wasdetermined. This was easily be done by phase angle measurements obtainedusing different concentrations of DDCI analyte as described below, or bydetermining the relative phase as a function of the modulation frequencyΩ. The standard deviation of phase angle measurement depended stronglyon the readout rate and the intensity of the photon signal and wastypically around 0.05 degrees. It was ultimately restricted by theinstrumental limit of the lock-in amplifier (which in the present casewas 0.02° according to the manufacturer's specifications).

In the capillary electrophoresis measurements the fiber ends were joinedby a commercial 4-way microcross (Upchurch Scientific) instead of thetranslation stage. In the microcross, the two fiber ends (100/140 μmFiber Tech Optica AS100/140IRA) were inserted through opposing holes(150 μm) and the capillary ends (Polymicro Technologies 100/360 μm) wereinserted through the other two holes. In this experiment the modulationfrequency was 150 kHz, the length of the fiber loop was about 65 m, andthe size of the gap between the fiber ends was calculated to be 31 μm.

Results

FIG. 2 shows a typical trace of the time dependence of the intensitydetected by the PMT. A driving voltage modulated at 200 kHz was suppliedto the laser diode and also used as a reference signal. The reference isshown as a solid curve and the phase shift of the emitted light can beeasily seen. The transient signal and reference were connected to thelock-in amplifier, which averaged the phase angle measurement over aperiod of 100 ms (variable from 1 ms to 1 s).

When the clear DMSO solvent was replaced with a solution of DDCI in DMSOat different concentrations, the phase angle was used to determine thering-down time and hence the absorption in the sample. DDCI is notoptically stable and its absorption band will shift into the visibleregion after prolonged exposure to the light. Therefore the phase-angleaverages of only the first 20 s were used in FIG. 3.

As mentioned above, there exists an offset to the phase angle arisingfrom inevitable time-delays in the electronic signal transmission in theelectronic components, the cables, and the laser diode, as well as lighttransmission in the power delivery fiber. This offset can readily beaccounted for by introducing the offset angle, φ₀, into Equation (3),resulting inφ_(m)=φ_(o)−arctan(Ωτ)  (5)where φ_(m) is the phase angle measured by the lock-in amplifier.Substituting the photon lifetime (Brown et al. 2002)

$\begin{matrix}{\tau = \frac{L}{c_{0}\left( {{- {\ln\left( T_{sp} \right)}} + {\alpha\; L} + {ɛ_{DMSO}d} + {ɛ_{{DDCI}\;}{Cd}}} \right)}} & (6)\end{matrix}$into Equation (5), one obtains

$\begin{matrix}{\phi_{m} = {\phi_{0} - {\arctan\frac{\Omega\; L}{c_{0}\left( {{- {\ln\left( T_{sp} \right)}} + {\alpha\; L} + {ɛ_{DMSO}d} + {ɛ_{{DDCI}\;}{Cd}}} \right)}}}} & (7)\end{matrix}$

Here ε_(DDCI)=3.343×10⁵ (M cm)⁻¹ is the extinction coefficient of DDCI(given with respect to base e) at its peak absorption wavelength of 825nm, C_(DDCI) is its concentration (M), and d is the width of the cavityformed by the two fiber ends. A similar term is introduced to accountfor the absorption of the solvent.

To simplify the model one can combine the optical loss from the splicealignment, the absorption of the fiber and the solvent into a singleexpression A₀. This term represents all optical loss processes otherthan the analyte's absorption and is constant for a given flow system.

$\begin{matrix}{\phi_{m} = {\phi_{0} - {{arc}\;\tan\frac{\Omega\; L}{c_{0}\left( {A_{0} + {ɛ\;{Cd}}} \right)}}}} & (8)\end{matrix}$

As can be seen from Equation (8), the phase angle dependence onconcentration is not linear. An advantage of this non-linearity is thatphase-angle measurements are more sensitive at low concentrations. Theoffset angle φ₀ can be determined from a linear fit using Equation (8)and the fact that −c tan(φ_(m)−φ₀) is proportional to the concentration.Eventually, this procedure will be done when calibrating the detector.For small concentrations, the determination of the offset angle is notnecessary since φ_(m) changes approximately linearly with concentration.

In Equation (8), only φ₀, A₀, and d were unknown and were determined byfitting (FIG. 3) to give φ₀=−36.3°, A₀=0.60, and d=42 μm. For a 26.4 mfiber loop, the ring-down time of τ=224 ns without DDCI is thereforemuch shorter than expected for the light decay in the fiber core (τ˜1μs), which implicates that the detected photons were scattered not onlyfrom the fiber core, but also from the fiber cladding. Furthermore,high-order core and cladding modes, which were biased against inprevious pulsed fiber loop ring-down spectroscopy (FLRDS) experiments bygating the detector (Brown et al., 2002), gave rise to strong and fastintensity decays within the first 100 ns.

Because the lock-in amplifier can only provide one averaged phase angle,measurements at a number of modulation frequencies need to be made tocharacterize the components associated with the different optical decayprocesses. However, since one needs to Fourier transform these frequencydomain measurements in real time, the time-resolution of the measurementis affected in an on-line detector. A simpler way to solve the problemof multiple optical decays with different time constants is to increasethe length of the fiber loop and thereby enhance the relative losses ofthe high-order modes and cladding modes over the long-lived low-ordercore modes.

FIG. 4 is a demonstration of the greatly increased time-response of thephase shift FLRDS (PS-FLRDS) system over pulsed FLRDS. Here, drops ofDDCI with different concentration were added alternately with pure DMSOsolvent between the two fiber ends. The rapid change in phase anglebetween −52 deg to −40 deg and −35 deg was used together with thecalibration of FIG. 4 to obtain quantitative and time-resolvedconcentration transients. From the figure the time response was obtainedfrom the onset of the leading edge of each peak, and was better than 200ms. We consider this a lower limit since the time needed to displace theexisting solution is on a similar timescale and may determine the timeresolution one can obtain in this experiment.

For the capillary electrophoresis experiment a microcross was used tocouple the fiber ends, and the concentration dependence of phase angleis shown in FIG. 3. A fit of the experimental data using Equation (8)yielded A₀=0.66, φ₀=−23.3°, and d=31 μm. The background opticaltransmission, A₀, was slightly higher compared to the value of A₀=0.66for the 400/440 μm fiber experiment, indicating that one can achievefair fiber-fiber coupling efficiencies with a simple and inexpensivecommercial microcross.

To test the capabilities of FLRDS in a more realistic analyticalenvironment, a fast transient peak was obtained in two ways. First asyringe was used to inject the solution into the capillary, and FIG. 5shows the measured change of phase angle when two differentconcentrations and different sizes of plugs of solution were used.Secondly, a capillary electrophoresis separation of two dyes, ADS805WSand ADS830WS (American Dye Source, Inc., chemical formulaeC₃₆H₄₄ClN₂O₆S₂Na and C₄₆H₅₁ClN₂O₆S₂, respectively) was undertaken, andresulted in an electrochromatogram that shows the retention times of notonly the original dyes, but also of the degradation products (FIG. 6).

DISCUSSION

To determine the detection limit, the derivative of Equation (8) wascalculated as follows:

$\begin{matrix}{{\frac{\mathbb{d}\phi}{\mathbb{d}C}}_{C = 0} = \frac{\Omega\; L\; ɛ\; d\; c_{0}}{{c_{0}^{2}A_{0}^{2}} + {\Omega^{2}L^{2}}}} & (9)\end{matrix}$If the detection limit (DL) is defined as 3σ_(c), then

$\begin{matrix}{{DL} = {\frac{{c_{0}^{2}A_{0}^{2}} + {\Omega^{2}L^{2}}}{\Omega\; L\; ɛ\; d\; c_{0}}3\sigma_{\phi}}} & (10)\end{matrix}$

For an uncertainty in the phase angle measurement of σ_(φ)=0.05°, adetection limit of 6 μM was calculated. This value is comparable to thelowest concentrations of 30 μM and 15 μM shown in FIG. 4. Given thesample volumes of 5 nL and 240 pL respectively, these concentrationscorrespond to 150×10⁻¹⁵ mol and 3×10⁻¹⁵ mol, respectively. PS-FLRDS thusenables absorption detection of about 2 billion molecules within about100 ms.

A number of improvements can be made to lower the detection limit. Theaccuracy of phase angle measurements affects the detection limit. σ_(φ)is mainly decided by the amount of signal fed into the lock-in amplifieras determined by the coupling efficiency. Improving the signal fractionfrom the core over the cladding modes increases A₀, whereas tuning tothe peak absorption wavelength (ε) of the analyte and optimizing the gapsize, d, and fiber length, L, further helps to achieve a lower detectionlimit.

The data collection rate was 10 Hz and the time constant of the lock-inamplifier was therefore set to 100 ms. Should it be required one canwork at considerably higher readout rates—limited ultimately by themodulation frequency to 10 μs—but for microfluidic devices, a dataacquisition rate of 10-100 Hz is sufficient to distinguish the transientpeaks.

We note that many of the disadvantages of phase shift CRDS measurementsdo not play a large role in PS-FLRDS. While the effective ring-down time(RDT) is an average of the RDTs in the fiber core and cladding, andtherefore considerably smaller than that expected from RDT in the fibercore only, its change with concentration is predictable and can beunderstood using Equation (6). Furthermore, since a large number ofmodes is excited in a multimode fiber, one does not need to be concernedwith mode-beating or mode-build-up effects. Finally, FLRDS is not somuch a tool for the measurement of very weak (strongly forbidden)transitions or for extremely dilute samples as for the determination ofsub-millimolar concentrations in small liquid samples. In thisapplication one would use a calibration curve together with anindependent measurement of the absorption cross section in a largesample to determine the concentration. Therefore, a possible inaccuracyin the effective ring-down time would be corrected for using the phaseshift technique of the invention.

CONCLUSIONS

In a conventional pulsed laser (10-100 Hz) FLRDS measurement, onering-down time measurement takes about 30-100 s while the effective dataacquisition takes only a small fraction of that time. PS-FLRDS improvesthe duty cycle and data acquisition rate, and thus enables real-timemeasurements in analytical environments. As demonstrated above, PS-FLRDSis suitable as an online absorption detector for capillaryelectrophoresis with time resolution of 10-100 ms and detection limit inthe micromolar concentration range.

PS-FLRDS is compact and inexpensive. In principle, one could integrateall electronic components, i.e., the function generator, diode laserdriver, photodiode circuit, and phase-detector into a single board witha computer interface, thereby reducing radio frequency interference animproving performance. PS-FLRDS is also robust and sensitive. Thequasi-continuous measurement of the phase angle eliminates softwareaveraging and exponential fitting, but does not permit a separatecharacterization of the competing optical loss processes. While this didnot pose an immediate problem in this example, fast optical decays incladding modes and lossy core modes will have to be dealt with as thedetection limit of FLRDS is reduced. Commercial fiber-fiber couplers canbe used to deliver the light signal into the optical loop as well as fordetection of its intensity in the loop. They will likely reduce thefraction of light travelling through cladding modes, and with highercoupling efficiencies also increase the accuracy with which the phaseangle can be measured.

In certain applications absorption detection at wavelengths shorter than300 nm may be desired; however, transmission of commercial fibers isgreatly reduced at wavelengths shorter than 600 nm. Example 2illustrates an experimental arrangement that conceptually permits FLRDSdetection of molecules in the spectral region used fortelecommunications. At about 1.6 μm optical waveguides have goodtransmission, and many biological molecules show absorption bands due tovibrational overtone transitions in this frequency range, which may beinterrogated using FLRDS.

Example 2 Near Infrared Phase Shift Optical-Loop Measurement of OpticalLosses

Introduction

In the fiber loop measurement scheme described in Example 1, there is alimit in the spectral range to which the technique may be applied. Thislimit is given by the use of the optical waveguide combined with thephotomultiplier detector. The absorption spectrum of a typical fiberoptic waveguide has a maximum transmission centered around two spectralregions near 1.35 μm and 1.5 μm, which are separated by a strongabsorption peak due to excitation of OH overtone vibrations in thewaveguide material. A third transmission window exists near 800 nm. Thetransmission then decreases dramatically as the wavelength of the guidedwave is decreased from the near infrared (NIR) through the visibleregion into the ultraviolet (UV). In example 1 the photodetector was aphotomultiplier tube with a detection efficiency whose wavelengthdependence is the inverse of the fiber transmission curve. The detectorwas most sensitive in the UV and visible region, with decreasingsensitivity in the near IR, until at about 850 nm the detectionefficiency decreased to near zero. The combination of these twowavelength response curves limits the experiment to a wavelength windowfrom about 780 nm to about 830 nm, and consequently many experiments todate have been conducted on sample dyes that show strong absorptionfeatures in this spectral region.

In this example an alternative excitation source and photodetector wereused to exploit the much higher optical transmission of the fiber opticcable in the 1.55 μm wavelength region. As shown in FIG. 7, using atunable (1.5-1.62 μm) NIR laser source 20 (ANDO), single mode fibers(Fiberguide), directional couplers 22, 24 (SENKO and Fiber metric), andeither an In—Ga—As photodiode 26 (Thorlabs) or an optical spectrumanalyser (Agilent 86142B), a 4 m fiber loop setup was realized, whichenabled the determination of optical losses of radiation travellingthrough the fiber core independently from the losses by the cladding.The phase angle φ_(m) was measured as a function of the angularmodulation frequency Ω of a broadband excitation source (Δv=200 MHz),and the optical decay constant τ=840 ns and the offset phase angleφ₀=44° were determined from the slope (FIG. 8). When using a narrow bandlight source (Δv=200 kHz) the resultant numbers were very similar,indicating that the longitudinal mode structure in the fiber loop waseither fluctuating very quickly and/or that the mode spacing was small.Note that in both experiments a 90:10 X-coupler was used to introducelight into the loop and a 99:1 tap was used to direct light out of theloop. Despite the fact that these optical devices have inherent lossesand that the two fusion splices by which they were connected into theloop also had optical losses, the loss per pass was calculated to bebelow 3%.

Similar experiments using a 99:1 X-coupler and 99:1 tap in a muchshorter loop of only L=1 m gave lower losses of only 0.7% (0.032 dB) perpass, indicating that there is a considerable amount of intensitybuild-up in the loop and also a higher than expected optical finesse.

An experimental arrangement such as this therefore allows for veryaccurate determination of optical losses in single mode optical devicessuch as, for example, Bragg gratings, couplers, and splices. Inaddition, the inventors contemplate introducing into the loop opticaldevices whose optical loss characteristics are modified as a function oftheir environment, so as to measure and monitor environmental variables,for example. Environmental variables that could be interrogated by sucha loop include, but are not limited to, temperature (e.g., as it affectsthe transmission/reflection characteristics of a Bragg grating), strainon the fiber, absorption spectrum of the evanescent wave, and refractiveindex of a surrounding medium. We note that wavelengths around 1.5 μmare particularly suited for unspecific detection of many organicmolecules through the vibrational overtone absorption bands of the CH,NH and OH stretching vibrations.

Example 3 Use of PS-FLRDS as a Fiberoptic Sensor for Mechanical Strain

An experimental setup similar to that described in Example 2 was used toinvestigate the effect of deformation on the optical waveguide loop. A10 m loop of 125 μm single mode fiber (Fiberguide) was formed bysplicing the fiber ends using a fusion splicer. Using a tunable(1.5-1.62 μm) NIR laser source (ANDO) which was coupled into the loopusing a 99.5:0.5 directional coupler (Lightel), and an inline powermonitor (EigenLight M160) connected to a lock-in amplifier, opticallosses of radiation traveling through the fiber core were determinedindependently from the losses caused by the cladding. The phase angleφ_(m) was measured at a fixed angular modulation frequency Ω of abroadband excitation source (Δv=200 MHz). Note that in this experiment a99.5:0.5 X-coupler was used to introduce light into the loop which wasdetected by the inline power monitor contained within the fiber loop.Additional loss was introduced by bending a section of the waveguideloop using a custom made strain sensor.

As shown in FIG. 9A, the strain sensor consisted of a sandwich of twoparallel arrays 100, 200 of cylinders 110, 210, each cylinder beingabout 12.5 mm in diameter, arranged such that the longitudinal axes ofthe cylinders of the two arrays were parallel but offset, and theoptical fiber. The fiber 120 was placed between the arrays perpendicularto the longitudinal axes of the cylinders. Compressing the waveguidebetween the two arrays caused the fiber to bend in a periodic nature asset by the size and spacing of the cylinders, and with a bending radiusthat depended on the force applied to the cylinder arrays. Impulse(time) response of the strain sensor was determined by dropping a 200 gweight 150 from a height of 3 cm onto the sensor.

The ring-down time (phase angle) and impulse (time) response of thestrain sensor are shown in FIGS. 9B and 9C, respectively. In particular,FIG. 9B shows that ring-down time decreased with increasing load,indicating that the optical loss increased with increasing deformationof the fiber. The minimum time response of the measurement was 10 ms,set by the 100 Hz data acquisition rate used. From FIG. 9C it can beseen that the response time of the device is not longer than this lowermeasurement limit.

Those skilled in the art will recognize, or be able to ascertain usingroutine experimentation, variations of the embodiments and examplesdescribed herein. Such variations are intended to be within the scope ofthe invention and are covered by the appended claims.

REFERENCES

-   Berden, G.; et al., 2000, Cavity ring-down spectroscopy:    Experimental schemes and applications, International Reviews in    Physical Chemistry 19:565.-   Brown, R. S.; et al., 2002, Fiber-loop ring-down spectroscopy, J.    Chem. Phys. 117:10444.-   DeMille, S.; et al., 2002, Comparison of CRDS to ICL-PAS and    phase-shift CRDS spectroscopies for the absolute intensities of C—H    (?v_(CH)=6) overtone absorptions, Chem. Phys. Left. 366:383.-   Engeln, R.; et al. 1996, Phase shift cavity ring down absorption    spectroscopy, Chem. Phys. Lett. 262:105.-   Hallock, A. J.; et al., 2002, Direct monitoring of absorption in    solution by cavity ring-down spectroscopy, Anal. Chem. 74:1741.-   Jakubinek, M.; et al., 2004, Configuration of ring-down    spectrometers for maximum sensitivity, Can. J. Chem. 82:873.-   Lewis, E.; et al., 2001, Phase shift cavity ring-down measurement of    C—H (?v=6) vibrational overtone absorption's Chem. Phys. Left.    334:357.-   Polynkin, P.; et al., 2004, Efficient and scalable side pumping    scheme for short high-power optical fiber lasers and amplifiers,    IEEE Photonics Technology Letters 16:2024.-   Romanini, D.; et al. 1993, Ring-down cavity absorption spectroscopy    of the very weak HCN overtone bands with six, seven, and eight    stretching quanta, J. Chem. Phys. 99:6287.-   Scherer, J. J.; et al., 1997, Cavity ring-down laser absorption    spectroscopy: History, development, and application to pulsed    molecular beams, Chemical Reviews 97:25.-   Stewart, G.; et al., 2001, An investigation of an optical fibre    amplifier loop for intra-cavity and ring-down cavity loss    measurements, Meas. Sci. Technol. 12:843.-   von Lerber, T.; et al.; 2002, Time constant extraction from noisy    cavity ring-down signals, Chem. Phys. Lett. 353:131.-   Xu, S.; et al., 2002, Cavity ring-down spectroscopy in the liquid    phase, Rev. Sci. Instr. 73:255.

1. A method for measuring one or more optical properties of a testmedium, comprising: providing a passive optical waveguide loop thatprovides a continuous path for a light signal launched into the loop totravel around the loop repeatedly, the loop adapted to accept a testmedium such that the light signal traveling around the loop interactswith the test medium each time the light signal travels around the loop;launching in the optical waveguide loop an intensity-modulated light forilluminating the loop with a light signal at an intensity modulationenvelope reference phase; detecting a phase of the intensity modulationenvelope of said light signal along the optical waveguide loop; andcomparing the detected phase of the intensity modulation envelope ofsaid light signal with the reference phase of the intensity modulationenvelope of the light signal; wherein a result of the comparison isindicative of one or more optical properties of the test medium.
 2. Themethod of claim 1, wherein the optical waveguide is an optical fiber. 3.The method of claim 1, wherein the waveguide loop is the test medium. 4.The method of claim 1, wherein the optical waveguide loop comprises acapillary channel for said test medium.
 5. The method of claim 1,wherein the test medium is exposed to an evanescent wave of light thatis guided by the optical waveguide loop.
 6. The method of claim 5,wherein the optical waveguide loop comprises a cladding, and the testmedium is in the cladding.
 7. The method of claim 1, wherein the opticalproperty is absorbance.
 8. The method of claim 1, wherein the light hasa wavelength selected from about 200 nm to about 2000 nm.
 9. The methodof claim 1, wherein the test medium is selected from a gas, a liquid,and a solid material.
 10. The method of claim 1, wherein the test mediumis a liquid.
 11. The method of claim 1, wherein the optical waveguideloop comprises a single-mode optical fiber, the method comprisinglaunching in the optical waveguide loop a single longitudinal mode of anintensity-modulated light; wherein the phase of the longitudinal mode isindicative of one or more optical properties of the test medium.
 12. Themethod of claim 11, further comprising measuring intensity of saidlongitudinal mode.
 13. The method of claim 1, wherein the test mediumcomprises a mechanical sensor for sensing a mechanical force, and theone or more optical properties of the test medium provide informationabout the mechanical force sensed by the mechanical sensor.
 14. Themethod of claim 13, wherein the mechanical force is selected from stressand strain.
 15. The method of claim 1, wherein the optical waveguideloop comprises a microfluidic device.
 16. The method of claim 1, whereinthe optical waveguide loop comprises a single-mode optical fiber. 17.The method of claim 1, wherein the optical waveguide loop comprises agrating.